The present invention relates to a type of down-the-hole percussive drilling hammer having a drilling bit with the ability to drill a hole with one outer diameter and then reduce its overall outer diameter for extraction from the hole through a pipe or casing that is inserted while the hole is being drilled. Insertion of a pipe into the ground is a common practice for many types of construction projects. For example, water wells use a casing for sealing contaminated surface water from potable deeper water. Sometimes the casing is only installed by under-reaming to prevent the hole from caving or collapsing into itself. Foundations commonly use a process called micropiling where a series or collection of smaller pipes are under-reamed into the ground and secured in place to create a larger foundation structure. In all applications, these products are known as under reamers.
There are many styles and types of available under reamers. Concentric systems typically use a “lost crown” or “ring bit” to create a kerf for the casing. While such systems are effective, they are costly to use for micropiling since the kerf-cutting ring bit is left (i.e., discarded) in the hole and because of the high number of piles associated with micropiling. Eccentric systems typically use a hinged wing or segment that rotates away or toward the pilot bit to create a kerf for casing clearance. Such systems have the operating cost advantage of not leaving a ring bit in the hole, but they do have operational limitations. For example, the hinged wing is rotated and held outward by a clockwise torque requiring contact with the borehole. If borehole contact is lost, it is likely that the wing will not stay in position, thus leaving an undersized hole through which the casing will not fit.
Regarding lug based systems, such lug systems use a series of radially extendable lugs. For example, in one prior art device, lugs are extended radially outwardly by making contact with the base of the borehole. A known limitations of such lugs is that they require constant contact with the bore hole bottom to remain in the extended configuration. Therefore, if the ground surface cannot supply ample back pressure to extend the lugs (such as when advancing through sand, voids or other similar conditions) the system will drill an undersized hole that will not allow the casing to clear the hole.
Therefore, it would be desirable to have a percussive down-the-hole drill hammer under reaming system that addresses the foregoing limitations of conventional down-the-hole under reamers.